Motor vehicle emission control system

ABSTRACT

Emission from motor vehicle exhausts of oxides of nitrogen, carbon monoxide, hydrocarbons, and aldehydes is reduced by a combination of enrichment, under off-idle operating conditions, of the fuel-air mixture fed to the engine, and an exhaust converter, e.g., a thermal exhaust reactor. The enrichment is brought about by a mechanical modification of the carburetor, and a selective carburetor control arrangement responsive to engine speed is utilized for enleanment of the fuel-air mixture at higher engine speeds.

United States Patent lnventor Paul E. Oberdorler, ,Ir.

Devon Township, Wllmlnton, Del. Appl. No. 38,176 Filed May I8, 1970Patented Aug. 17, 1971 Assignee Sun Oil Comply Philadelphia, Pa.

Contiuultlon'll-part ot application Ser. No. 6,832, Jan. 29, I970.

MOTOR VEHICLE EMBSION CONTROL SYSTEM 9 Clubs, 2 Drawing I1.

Primary ExaminerWendell E. Burns Arlomeys-George L. Church, Donald R.Johnson, Wilmer E.

McCorquodale, Jr. and Frank A. Rechif ABSTRACT: Emission from motorvehicle exhausts of oxides of nitrogen, carbon monoxide, hydrocarbons,and aldehyde: is reduced by a combination of enrichment, under ofl-idleU.S. 60/29, operating conditions, of the fuel-air mixture fed to theengine, 60/30, 123/97 B, l23/l 19 D, l23/l24 arid an exhaust converter,e.g., a thermal exhaust reactor. The Int. Cl ..F02b 75/ 10, enrichmentis brought about by a mechanical modification of F02m 23/00, F02m 23/04the carburetor, and a selective carburetor control arrange- Field ofSearch 123/] 24 B, ment responsive to engine speed is utilized foren1eanment of 1 19 D, 97 B; 60/29, 30 the fuel-air mixture at higherengine speeds.

C O N T R O L ASSEMBLY 5 CARBURETOR 1 2 I l' HERMAL THERMAL To HEfiiREAcToR REACTOR To j z a PATENTED IUGITIHYI 3,599,426

CONTROL ASSEMBLY FIGI.

CARBURETOR Z THERMAL 7 To MUFFLER TO MUFFLER ETC. MFIEACTOR ETC AIR-FUELMIXTURE FIG. 2, I

' 7 CALIBRATED CLEAN Alp A AIR IN (FROM RESTRICTION I AIR CLEANER)CARBURETOR MAIN WELL VIA IDLE RESTRICTION To ENGINE INTAKE MANIFOLD+I2V. IGNITION PULSES IN (FROM COIL) INVENTOR f the control'assemb 3.According o this MOTOR VEHICLE EMISSION CONTROL SYSTEM This applicationis a continuation-in-part of my copending application, Ser. No. 6,832,filed Jan. 29, 1970.

This invention relates to controlling emissions from the exhausts ofmotor vehicles powered by gasoline-fueled internal combustion engines,and more particularly to controlling these emissions in such a way as toreduce certain air pollutants (e.g., smog-forming pollutants) commonlyfound in motor vehicle exhaust gases.

This application is related in a general way to my copendingapplication, Ser. No. 889,252, filed Dec. 30, I969.

It has been recognized that certain types of atmospheric pollutantswhich are commonly found in exhaust emissions from motor vehicles shouldbe reduced; in fact, Federal standards have been set for maximum exhaustemissions of hydrocarbons and carbon monoxide, on a mass emission basis,and standards for emissions of oxides of nitrogen (denoted hereinafterby NO,) are now under active consideration. It is also desirable toreduce exhaust emissions of aldehydes.

An object of this invention is to provide a novel motor vehicle emissioncontrol system.

Another object is to provide an emission control system for automobileswhich does not adversely affect the driveability of the automobile.

A further object is to provide an automotive emission control systemwhich gives results showing substantial improvement overfactory-equipped automobiles.

The objects of this invention areaccomplished, briefly, in the followingmanner: In the internal combustion engine of a motor vehicle which usesfull-boiling gasoline as a fuel, the carburetor is modified mechanicallyto produce an enrichment of the fuel-air mixture fed to the engine underoffidle operating conditions, and a selective carburetor enleanmentarrangement (which enleans the fuel-air mixture fed to the engine athigher engine or vehicle speeds) is utilized. One or more converters areutilized on the engine exhaust. The enrichment suppresses formation ofNO, and also reduces the emission of aldehydes, while the enleanmentminimizes the drop in fuel economy. The downstream converters removecombustible gases such as hydrocarbons and carbon monoxide from theexhaust by conversion thereof to harmless substances.

A detailed description of the invention follows, taken in conjunctionwith the accompanying drawing, wherein:

FIG. I is a diagrammatic illustration showing the principal elements ofa motor vehicle emission control system (exhaust pollutant reductionsystem) according to this invention; and

FIG. 2 is a schematic illustration of a selective carburetor controlarrangement which may be utilized in the invention.

Refer first to FIG. I. A V-type or V-design internal combustion engine,which uses full-boiling gasoline (a so-called fossil fuel, or ahydrocarbon fuel) as a fuel, is quite schematically represented bynumeral 1. A carburetor 2, of somewhat conventional construction butmodified as hereinafter described (to achieve an enrichment action plusa selective enleanment action), supplies a fuel-air mixture to the twocylinder banks of engine 1. The selective enleanment action of thecarburetor 2 is effected by means of a control assembly 3, hereinafterdescribed.

An exhaust converter, preferably a thermal reactor 4 of the typedescribed more fully hereinafter, is mounted in the engine compartmentof the motor vehicle (e.g., automobile) to receive the effluent(exhaust) from one bank of cylinders, and a duplicate unit 4', alsomounted in the engine compartment, receives the effluent from the othercylinder bank. The outlet connections 5 and 6, respectively, of theconverters 4 and 4' are connected to the conventional exhaust system ofthe automobile, including a muffler or mufflers, tailpipes, etc.

Refer now to FIG. 2, which shows, somewhat schematically, a portion ofthe carburetor 2, together with adjuncts including tions (or, to statethis another way, during urban-type, lightload accelerations involved inthe driving cycle). In this connection, it has been determined that thebulk of urban N0, emissions occurs during these light-loadaccelerations.

In FIG. 2, a portion of one of the main barrels or bores of afour-barrel carburetor is illustrated at 7, a throttle valve 8 ofconventional type being pivotally mounted within this bore. Thewide-open or full-throttle position of this valve is illustrated insolid lines, while the off-idle position of this valve is.

illustrated in dotted lines. In the latter position, main bore 7 isopened only very slightly. The lower end of main bore 7 leads to theintake manifold of the engine, while above the throttle valve 8 thereare venturi passages (not shown) which feed gasoline from the carburetorbowl into the bore 7, to be mixed with air and fed (when the throttlevalve 8 is opened sufficiently) into the engine intake manifold. An idlemixture needle hole 22, in which there is an adjustable needle valve(not shown) operated by an idle adjustment screw, opens into the mainbore 7 some little distance below the dotted-line position of valve 8,such that an air-fuel mixture may be fed (by way of hole 22) into thebore 7 below the throttle valve 8 (and thus into the engine intakemanifold) when the latter is in its fully closed or idle" position. Thisfully closed or idle" position of the throttle valve 8 is one whereinthe valve closes off bore 7 somewhat more completely than it does inthedotted-line off-idle position illustrated.

A pair of vertically spaced idle discharge ports 9 and 9' open into themain bore 7 somewhat above the idle mixture needle hole 22, such that afuel-air mixture can be fed by way of these ports into the bore 7 belowthe throttle valve 8 (and thus into the engine intake manifold) when thelatter is in its dotted-line off-idle" position. In some carburetors, asingle idle discharge slot may be provided instead of the two separateidle discharge ports 9 and 9. The fuel for this air-fuel mixture (aswell as the fuel for the fuel-air mixture fed through hole 22) isoriginally derived from the carburetor bowl, and flows from thecarburetor main well by way of idle tube 23 containing therein an idletube restriction (not shown), mixing with air along the way, to ports 9and 9'. All of the elements mentioned in connection with the carburetor,such as the bowl, the main well, the main bore, the throttle valve, theidle tube, etc., are contained in the body of the carburetor, asintegral parts thereof.

In an embodiment of this invention which was actually built forexperimental purposes and successfully tested, the original factorycarburetor was mechanically modified by enlarging the idle tuberestriction or restrictions (there may be more than one of theserestrictions in the carburetor), by appropriate machining. (Of course,on a commercial or mass production basis, the original factorycarburetor itself would be redesigned to effect these enlargements, sothat no separate machining would be necessary.) The enlarging of theidle tube restriction (that is, the increasing of its diameter) permitsan enriched idle with leaner idle adjustment screw turnout, which inturn creates a considerably enriched off-idle operating condition. Thisenrichment of the air-fuel mixture fed to the engine intake manifoldduring off-idle conditions (as a result of the tailoring of the idletube restriction) brings about a reduction in NO, emissions during thelight-load accelerations involved in the driving cycle. Additionalenrichment beyond (above) off-idle speeds can be obtained by enrichmentof the main metering jets (e.g., by reduction of the primary meteringrod diameter). This may or may not be necessary, depending on enginesize, car application, etc.

Since enrichment of the air-fuel mixture fed to the intake manifold ismade to occur in the above-described manner, and would tend to have aneffect throughout all engine operating conditions (that is, throughoutall positions of throttle valve 8,

\ melee armies fuel economy at higher speed cruise conditions (highwayconditions), where exhaust emissions are less of a problem, thecarburetor 2 is controlled to provide a selective enleanment action,which results in an enleanment of the air-fuel mixture going to theintake manifold at all vehicle speeds a above a chosen preset speed, forexample approximately 35 mph This enleanment action is produced by theselective carburetor control arrangement 3, in response to engine speed.It may be stated that the enleanment action is produced byspeedresponsive (rpm-controlled) throttle bore air ports.

One or more air ports 10, opening into the carburetor main bore 7 abovethe main throttle valve 8, are provided. A line (tubing or hose) 11extends from port or ports to a source of clean air at atmosphericpressure, such as the downstream side of the carburetor air cleaner. Asolenoid-operated valve 12 and a calibrated air restriction 13 (whichmay, in the alter native, be an adjustable needle valve) are located inline 11. When valve 12 is open, clean air is drawn by manifold vacuumthrough air line 11 and restriction 13 into the carburetor main bore 7(assuming throttle valve 8 is opened sufficiently), this additional flowof air causing an increase in the air-fuel ratio of the mixture going tothe intake mainfold, and thus an enleanment of this mixture.

Pulses derived from the ignition coil of the engine, the repetition rateof which is of course directly proportional to the speed of the engine,are fed at 14 into the input of a pulse counting and trigger unit 15,which rectifies these pulses and applies them to an internal triggercircuit set to produce a signal voltage across the output leads 16 ofunit 15 when the input pulse repetition rate corresponds to apredetermined engine speed, say that corresponding to approximately 35mph. vehicle speed, though not limited thereto.

The unit 15 may comprise a commercially available tachometer-typesolid-state circuit, such as are often used in racing, as overspeedindicators on engines. These units are responsive to engine speed andproduce an output signal voltage when a certain engine speed is reached.

The output leads 16 are connected to the coil of a relay 17 having apair of normally closed contacts 18 which when closed supply 12 volts(derived from the car battery) to a solenoid 19 which operates the valve12 in'the air line 11 for supplying air to the carburetor bore 7. Thearrangement is such that when solenoid 19 is deenergized valve 12 isopen, supplying air to the bore 7. It is here pointed out that elements14 18 essentially make up the control assembly 3 of FIG. 1.

At or above the preset engine speed corresponding (by way of example) toapproximately 35 m.p.h. vehicle speed, which latter may be thought of asthe trigger speed," the repetition rate of the ignition pulses fed intounit 15 becomes high enough to actuate the trigger circuit of this unit,causing a signal voltage to appear on leads 16. This energizes relay 17to open its contacts 18, deenergizing solenoid l9 and opening valve 12to supply air to carburetor main bore 7. These conditions are the onesillustrated in FIG. 2. The opening of valve 12 causes clean air to bedrawn through air line 11 into the bore 7, increasing the air-fuel ratioand enleaning the air-fuel mixture going to the engine intake. Aspreviously described, this selective embodiment, at higher speedconditions, tends to restore fuel economy under such conditions.

Below the trigger speed," there is no signal voltage on leads l6, relayI7 is unenergized, contacts 18 are closed, and solenoid 19 is energized,which closes the valve [2, cutting off the supply of additional air tocarburetor bore 7. Therefore, no enleanment of the air-fuel mixturegoing to the engine intake occurs under these conditions.

It may be more expedient to utilize an alternative to the special airports 10 in bore 7. A vent plate which has an arrangement of transverseholes extending from the outside of the plate to the carburetor bore,may be utilized as a mounting for the carburetor, in the manner of athick gasket. The outer ends orv termini of the holes would be coupledto a source of 2' and 'a calibrated air t i it til.

liQnrlli .1;

Refer again to FIG. 1. As previously stated, the emission control systemof this invention includes a pair of exhaust converters (preferablythermal reactors) 4 and 4' for acting upon the carbon monoxide andhydrocarbons (as well as other combustibles such as aldehydes) whichappear in the effluent from the combustion chambers or cylinders of theengine. A thermal reactor 4 is connected to the four exhaust ports ofone cylinder bank of the V-type engine 1, and an exactly similar thermalreactor 4' is connected to the four exhaust ports of the other cylinderbank of the engine. The thermal reactors 4 and 4' will not be describedin detail herein, since per so they form no part of the presentinvention; by way of example, they may be of the construction disclosedin the Rosenlund- Douthit US. Pat. No. 3,4l3,803. The patented reactoris of a double (concentric) hollow tubular construction, with nocatalyst; the outer tube has relatively thin layers of heat-insulatingmaterial on its inner faces. Each reactor has four inlet connections,one for each of the four cylinders of the corresponding cylinder bank,and a single outlet connection; these reactors may therefore be termedthermal exhaust manifold reactors. For reactor 4, one of the inletconnections is schematically indicated at 20 and the outlet connectionis schematically indicated at 5; this latter connection extends to themuffler, etc. of the dual exhaust system of the automobile. For reactor4', an inlet connection is schematically indicated at 21 and the outletconnection at 6; this latter connection extends to the muffler, etc. ofthe automobiles dual exhaust system.

Although not illustrated in the drawing, an air injection system(secondary air delivery system) is preferably used in conjunction withthe thermal reactors 4 and 4'. This system utilizes an air delivery pumpbelt-driven from the engine and having a filter and silencer on itsintake. Air from this pump is fed continuously, by means of individualdelivery tubes and channels, to a point just underneath each exhaustvalve, outside each combustion chamber or cylinder, so that it isintroduced at each exhaust valve but just beyond the combustion chambersor cylinders, on the way to the reactors 4 and 4'.

As described in the above-identified patent, oxidation (i.e.,combustion) of hydrocarbons and of carbon monoxide contained in theexhaust gases (effluent from the combustion chambers of the engine)takes place in the thermal reactors 4 and 4, the oxidation reactionbeing enhanced by various types of turbulencewhich are caused to occurin the reactors. In addition to the combustion of the carbon monoxideand hydrocarbons which takes place in the reactors, combustion oroxidation of aldehydes, which may be left over in the effluent from thecombustion chambers of the engine, is postulated to also take place inthe thermal reactors, since these al dehydes are combustible substances.

It may be noted here that the enrichment described in connection withFIG. 2, and also the selective enleanment arrangement, operate on theinduction system or intake of the engine, while the thermal reactorsoperate on the exhaust of the engine. However, there is an importantinteraction or interplay between the two, as will now be explained. Thecomplete overall system includes two combustion devices which must workin harmony, the first being the engine itself and the second being thethermal reactors. First, the carburetor is optimized (by enrichment) toreduce to a minimum the N0, (and also the aldehydes, which decrease withricher mixtures), in keeping with good engine performance, economy, anddriveability and then the secondary air delivery system (air in effectinjected into the thermal reactors) must be tailored to give the properair-fuel ratio to the reactors, such that these latter will (a)completely burn the carbon monoxide and hydrocarbons; (b) at the sametime, not be burned up themselves.

By way of example, in order to minimize the NO, and the aldehydes in thecombustion chamber (by way of carburetor enss bss s m hmeiit action,with more fuel going into the cylinders). This means that more carbon'monoxide is being sent as fuel to the thermal reactors. Without moreair, the reactors would not burn this properly due to either improperchemical balance or improper combustion temperatures. The ratio-of C0 toair fed to the reactors is critical since an optimum combustiontemperature is desired for emissions reduction; at the same time, thecombustion must be limited so that the temperature does not get so highthat the reactors burn up. So, the amount of air that is injected intothe inlet of the reactors (by means of the secondary air deliverysystem) must be readjusted to compensate for the action that has beeneffected in the carburetor to produce a certain amount of carbonmonoxide and at the same time the amount of CO produced by enrichment(and thus the level of enrichment) must be carefully balanced.

As has been described previously, the carburetor enrichment (at off-idlecondition) employed in this invention requires the supplyingofadditional air to the reactors (by means of the secondary air deliverysystem described), over and above what would be, supplied to thereactors absent any enrichment, in order to properly oxidize or burn thecarbon monoxide and hydrocarbons in the combustion chamber effluent.This means that when the enrichment is in effect cut off by thecutting-in of the selective enleanment arrangement, there will be anexcess-air condition which prevents the reactors 4 and 4' fromoverheating. That is,at higher speed conditions more air is supplied tothe reactors than they need; they still function to reduce thehydrocarbons, carbon monoxide, and aldehydes to a low level, but theextra air preventsthe reactor temperature from continuing to rise, thusrelieving any tendency to burn out the reactors.

Because of the enrichment employed in the emission control system ofthis invention, the engine requires slightly more gasoline. That is tosay, with the present system there is an inherent reduction in fueleconomy. At 30 m.p.h., it was found that there was a loss of about l3percent in fuel economy (miles per gallon), though in highway driving(60 m.p.h.) the fuel economy reduction is rather small (due primarily tothe selective enleanment of the air-fuel mixture at higher speeds, whichis characteristic of the carburetor control of FIG. 2). However, eventhis (about l3 percent) reduction or loss in fuel economy is small ascompared to that encountered in other emission control systems.

Additional spark retard is commonly used with thermal or catalyticreactors, to obtain rapid warmup and additional reactor temperature,which is said to provide a reduced level of emissions (as compared tothat obtained with no additional retard).- However, thisadditional sparkretard (.if used indiscriminately) carries with it a severe drop in fueleconomy, a drop in the car performance or driveability and an increasein underhood temperatures. It is pointed out that no additional sparkretard is utilized in the present system; the standard ignition systemis used, retaining the standard advance curve and initial timing. Thus,the disadvantages of additional spark retard can be obviated. Additionalspark retard can be used with the present system to afford additionalreduction of emissions (at the above described expense, however), and isnot excluded from the scope of this invention.

In the past, exhaust gas recirculation has often been employed to reducethe emission of N0,. However, this has a definite and substantialadverse effect on the car performance or driveability also, the control(valving, rerouting, or switching) of hot exhaust gas is extremelydifficult and troublesome. It is pointed out that in the system of thepresent invention no exhaust gas recirculation is used, so that theproblems associated with such recirculation are avoided. Again, however,if additional N0 reduction were desired, some exhaust gas recirculationcould be used, but only a much lesser amount would be required, so thatperformance or driveability would not be unduly compromised.

It is again pointed out, however, that spark retard and exhaust gasrecirculation both result in an adverse effect on driveability Incontrast, in the emission control system of the present invention (usedwithout these alternative methods) the driveability in terms ofacceleration as well as other criteria such as startability, warmup,throttle response, etc. has remained excellent.

As previously described, the selective carburetor enleanment arrangementof FIG. 2 is responsive to engine speed, since ignition pulses are fedat 14 into unit 15. It may be more desirable, in some instances, to basethe carburetor control on vehicle speed, rather than on engine speed,such as in the case of a yehicle with an automatic transmission, whereinthere is slippage and wherein some fuel economy may be otherwise lostdue to that slippage. For vehicle speed, a small pulse generating deviceof some sort could be placed in the drive line, or in the transmissiontail shaft which operates the speedometer, in order to generate pulses(proportional to vehicle speed) for application to the input of unit 15.Mechanical vehicle speed sensing (not employing pulses) couldalternatively be used for controlling the selective carburetorenleanment arrangement. For this purpose, it is anticipated thatswitches of known type (responsive to transmis' sion pressure, or todrive line r.p.m.) could be employed to suitably control the solenoid19.

The following example, illustrating the novel motor vehicle emissioncontrol system of this invention and the results obtained therewith, isgiven without any intention that the invention be limited thereto. Thesampling and analysis of emission components were carried out in theprescribed manner according to the Federal test procedure, using theseven-mode cycle. In the absence of regulations, the NO, were determinedby the H.E.W. recommended sampling and subsequent Saltzmandetermination. The automotive vehicle utilized was a 1967 ChevroletCamaro, a sporty-type performance-oriented vehicle, with350-cubic-inch-displacement engine and manual transmission. This carutilized a single Rochester 4MV-A.I.R. Quadrajet carburetor, withenlarged idle tube restrictions according to the invention and withslightly enlarged main metering orifices, and also modified inaccordance with the principles set forth in FIG. 2 to provider.p.m.-controlled (speedcontrolled) throttle bore air ports. Two thermalexhaust reactors, of the type described in the aforementioned patent,were utilized.

Table 1, following, gives hot cycle test data obtained with thisvehicle.

As stated, table 1 displays only hot cycle test data. Table II,following, shows the data obtained running the composite cold startseven-mode cycle. It is interesting to note, in this latter table, thatduring the first cycle, with choke operation, the NO, were extremelylow, which would indicate a response to additional enrichment evencompared to the low levels ob tained throughout the test; unfortunately,this is offset by the increase in carbon monoxide and hydrocarbonsduring this first cycle.

Table II Cycle CO, I: l-IC, ppm. NO,, p.p.m.

Table IIContinued qauuttswm Composite 0.3] 7l I55 Aithough the emissioncontrol system of the invention has been described as including thethermal exhaust manifold reactors 4 and 4', this is merely the preferredembodiment. It is entirely possible touse, in place of these thermalreactors, catalytic converters of known type, and the use of such latterconverters is not outside the scope of the present invention. Catalyticconverters, when used in the system of the invention, will functioneffectively and efficiently in a manner similar to the thermal reactorspreviously described, which is to say that the former will also convertat least a part of the carbon monoxide and hydrocarbons contained in thecombustion chamber effluent to harmless substances.

The use of catalytic converters in the system of the present inventionmay be quite feasible and practical in view of the fact that one of themain drawbacks, if not the main drawback,

to their use (to wit, the poisoning of the catalyst by leaded Inconnection with catalytic converters, it will be appreciated that theprocedures of spark retard and of exhaust gas recirculation (both ofwhich were commonly used in the past, in conjunction with catalyticconverters, and both of which result in an adverse effect ondriveability are not at all necessary with the system of the presentinvention, and are ordinarily, and preferably, not utilized in thelatter system.

I claim:

l. A system for reducing the pollutants contained in the exhaust of amotor vehicle internal combustion engine using a fossil fuel comprising,in combination, means for enriching the fuel-air mixture fed to theintake of said engine during off-idle and low-flow main meteringoperating conditions, the enrichrnent being sufficient to reduce theemission from said engine of oxides of nitrogen; means responsive toengine or vehicle speed for automatically enleaning the fuel-air mixturefed to the engine intake at speeds at or above a preselected speed andunder nonclosed throttle conditions, and means acting upon the effluentfrom the combustion chambers of said engine for converting at least apart of the carbon monoxide and hydrocarbons contained in said effluentto harmless substances.

2. System according to claim 1, wherein the automatic enleaning meansacts to increase the air-to-fuel ratio of said mixture at the speeds andunder the throttle conditions stated.

3. System of claim 1, wherein said engine utilizes a carburetor forfeeding the fuel-air mixture to the engine intake, and wherein saidenriching means and said enleaning means both operate upon saidcarburetor.

4. System defined in claim 3, wherein said enleaning means comprisesactuatable means, separate from the main engine intake air supply, forsupplying air to the engine intake, and means responsive to the speed ofsaid engine or vehicle for actuating said separate air supplying means.

5. System recited in claim 1, wherein the last-mentioned means comprisesa thermal reactor operating to burn at least a part of the carbonmonoxide and hydrocarbons contained in said effluent.

6. System of claim 1, wherein the last-mentioned means comprises athermal reactorwith inlet connections and an outlet connection, meansconnecting the combustion chambers of the engine to said inletconnections, and means for connecting an exhaust pipe to said outletconnection.

7. For a motor vehicle internal combustion engine using a fossil fuel, amethod for controlling exhaust emissions which comprises enriching thefuel-air mixture fed to the intake of said engine under off-idle andlow-flow mainmetering operating conditions, thereby to reduce theemission from said en-

2. System according to claim 1, wherein the automatic enleaning meansacts to increase the air-to-fuel ratio of said mixture at the speeds andunder the throttle conditions stated.
 3. System of claim 1, wherein saidengine utilizes a carburetor for feeding the fuel-air mixture to theengine intake, and wherein said enriching means and said enleaning meansboth operate upon said carburetor.
 4. System defined in claim 3, whereinsaid enleaning means comprises actuatable means, separate from the mainengine intake air supply, for supplying air to the engine intake, andmeans responsive to the speed of said engine or vehicle for actuatingsaid separate air supplying means.
 5. System recited in claim 1, whereinthe last-mentioned means comprises a thermal reactor operating to burnat least a part of the carbon monoxide and hydrocarbons contained insaid effluent.
 6. System of claim 1, wherein the last-mentioned meanscomprises a thermal reactor with inlet connections and an outletconnection, means connecting the combustion chambers of the engine tosaid inlet connections, and means for connecting an exhaust pipe to saidoutlet connection.
 7. For a motor vehicle internal combustion engineusing a fossil fuel, a method for controlling exhaust emissions whichcomprises enriching the fuel-air mixture fed to the intake of saidengine under off-idle and low-flow main metering operating conditions,thereby to reduce the emission from said engine of oxides of nitrogen,selectively and automatically varying the composition of the mixture atengine or vehicle speeds at or above a preselected speed and undernonclosed throttle conditions, and burning combustible gases present inthe exhaust of said engine.
 8. Method of claim 7, wherein the varying iseffected selectively and automatically in response to the speed of saidengine or vehicle.
 9. Method of claim 7, wherein the varying comprisesenleaning the fuel-air mixture.